Traveling wave tubes



2 Sheets-Sheet 2 Feb. 16, 1960 E. c. DENCH TRAVELING WAVE TUBES Filed April-5, 1957 A EIEIUEIIII Hana E- T L c sum [36:11: YE V 2 4% MW; IM I; I; :3 .2

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ATTORNEY United States Patent TRAVELING WAVE TUBES Edward C. Dench, Needham, Mass., assignor to Raytheon Company, a corporation of Delaware Application April 5, 1957, Serial No. 650,999

8 Claims. (Cl. 315-3.6)

This invention relates to high frequency oscillating and amplifying devices which utilize the prolonged interaction between a stream of charged particles (electron beam) and a traveling electromagnetic wave guided by a retardation line. Devices of this type are generally designated beam traveling wave tubes.

The invention resides in a traveling wave tube incorporating a backward wave oscillator and a forward wave amplifier utilizing a delay line common to both the oscillator and the amplifier and in which attenuation means located in the oscillator section serves both the oscillator and amplifier. This structure obviates the necessity for employing output and input coupling means which are required where a separate oscillator is connected to a separate amplifier. By thus eliminating those coupling means, a source of difiiculty is removed, since those coupling means tend to introduce undesired reflections into their associated tubes. The invention permits the amplifier to be constructed without its own internal attenuation means and therefore a smaller structure is possible which has a gain equal to that of larger amplifiers which must employ internal attenuation.

In one embodiment of the invention, interaction between the electromagnetic Wave and the electron beam takes place in a transverse D.C. electric field having its lines of force at right angles to the direction of beam travel and a constant magnetic field having its lines of force at right angles to the direction of beam travel and at right angles to the D.C. electric field. This embodiment employing crossed electric and magnetic fields is termed an M type tube.

Another embodiment of the invention, termed an 0 type tube, employs a longitudinal electric field in which the interaction between the electromagnetic wave and the 1 electron beam takes place. The electric field in the 0 type tube extends in the direction of beam travel. The 0 type tube may also employ a magnetic field to assure focusing of the electron beam, but this magnetic field is not essential to the operation of 0 type tube in contradistinction to the M type where a magnetic field is essential for operation. With respect to structure, the 0 type tube is a simpler form but its power output is less than that attainable from a M tube of comparable size. The 0 tube operates upon the principle of transfer of kinetic energy from the beam to the traveling electromagnetic wave; the M tube operates upon the principle of transfer of potential energy from the beam to the traveling electromagnetic wave.

The invention, together with its principles of operation, will be better understood by reference to the following description when considered in connection with the drawings wherein:

Fig. 1 schematically depicts a rectilinear form of the invention utilizing crossed electric and magnetic fields;

Fig. 2 illustrates the invention embodied in a curvilinear form;

Fig. 3 delineates another species of the invention;

Patented Feb. 16, 1960 Fig. 4 is a graph exemplifying the dispersion characteristic of a delay line;

Fig. 5 symbolically illustrates the induction by an electron beam of wave energy in a delay line;

Figs. 6 and 7 symbolize traveling wave amplifier tubes; and

Figs. 8 and 9 symbolize traveling wave oscillator tubes.

In a backward wave oscillator the electron beam travels at a velocity associated with the phase velocity of a traveling wave space component moving in a direction opposite to that of the energy flow along a wave-propagating structure. In the forward wave amplifier, the electron beam travels at a velocity associated with the phase velocity of a traveling wave space component, moving in the same direction as the energy flow along a wave propagating structure. The wave propagating structure is a delay line of the periodic-filter type having suitable properties.

Properties of delay lines having a periodic structure may be summarized as follows.

Where energy at a given frequency is transmitted along an infinitely long periodic structure (i.e. a periodic structure that coincides with itself when translated any integral number of times by the pitch, p) the distribution of the electromagnetic field along the line has a periodic configuration and the complex amplitude E and E,, at two corresponding points A and A of two cells separated by n times the pitch are related by d .=p j

The electromagnetic field along a delay line having a periodic structure may be COIlSidfilfld" to be constituted by a superposition of traveling waves having phase velocities:

These waves, sometimes called space harmonics, which do not exist separately, have the same group velocity (in) 2 PI which is identical with the velocity of energy propagation along the line. Phase velocities v can be positive or negative according to the value of k. When v has a positive value in Equation 3, i.e., if -1r ip 1r for k 0, the phase velocity is in the same direction as the energy velocity; the corresponding wave is termed a direct wave. When V has a negative value (k 0), the phase velocity is in a direction opposite to the energy velocity and the corresponding wave is referred to as a backward wave or inverse wave. The fundamental wave (k=0) has the largest phase velocity and is direct or inverse according to the sign of 11/. The variation of 1/1 with frequency completely characterizes the dispersion of the delay line. Because phase velocities of the waves play an important role in the interaction between an electron beam and the electric field of a line having a periodic structure, it is convenient to represent graphically the dispersion of a delay line, not by the variation of ,b with frequency, but by the variation of the delay ratio" 2 k as a function of the wave length 7\. In the graph of Fig. 4, the abscissa represents increasing wavelength A in the direction of the arrow and the ordinate represents velocity v compared to the speed of light 0, i.e., c/v. A straight line passing through the origin is a line of constant 1/, i.e. \//=1r, and has a slope L 21rP 2 for reverse waves; the largest value that v can assume is c (the velocity of light), demonstrating that A, the dispersion, defined as the variation of the delay ratio with wavelength A, i.e.

d l) A= dx can be positive, negative, or zero for direct waves, with its maximum positive value equal to C t For reverse waves, the dispersion is necessarily positive and its minimum value is equal to Interaction of an electron beam with a direct wave or a reverse wave furnish two possibilities, the first leading normally to an amplifier and the second to an oscillator. It should be understood that both the direct and reverse waves propagate simultaneously in the delay line. This can be better understood by referring to Fig. and considering the coupling between an electron beam F and a delay line L as obtained by means of gaps in the line; the action of the field of these gaps causes the formation of electron bunches in the beam. When these bunches pass across a gap, two waves W and W are excited in the line, the energy of which propagates away from the gap, and the action of beam F on the line L is manifested by the superposition of waves which the beam excites as it traverses successive gaps. When the beam velocity equals the velocity of a direct wave, the waves W, (the energy of which propagates in the direction of the beam) excited in the different gaps have the same phase, and their efl'ects are additive; waves W have different phases and almost completely cancel one another. Energy carried by the line increases-in the direction of electron motion and the tube is normally used as an amplifier. This is the principle of traveling wave amplifier tubes.

When the beam velocity is equal to that of a reverse wave, waves W (the energy of which propagates in a direction counter to that of the beam) have the same phase and their effects are additive. In the latter case, the amplitude of the field carried by the line increases toward the origin of the beam, and energy created by the action of the electron beam travels in a direction opposite to it. This interaction is the principle of backward wave oscillator tubes, The frequency of oscillation of these tubes is determined by the average electron velocity in the beam which must be close to that of a reverse wave. Hence, the frequency of oscillation varies with velocity according to the dispersion curve of the particular delay line employed.

The two interaction mechanisms lead to four types of tubes, each of which can be classified in one of two categories of tubes designated respectively by O and M. The four tube types are indicated schematically in Figs. 6, 7, 8 and 9. In those figures, a delay line is designated by L, a sole electrode by P, an electron gun by G, an electron beam by F, a beam collector by C, a voltage source by V, attenuator means by a solid area A in the delay line, an input signal coupling by N, an output signal coupling by S, a magnetic field is indicated by B, and the symbol (2) signifies that the magnetic field is directed into the plane of the drawings. In the 0 tubes, Figs. 6 and 7, an electron beam F, accelerated by a voltage source V, penetrates into the interaction space adjacent to delay line L, where no continuous field is applied, with the exception of a longitudinal magnetic field designed to keep the beam parallel. In these tubes, the R.F. field acts essentially through its longitudinal component, which, after having caused a velocity modulation in the beam, restrains the electron bunches which are formed by the action of this velocity modulation.

In the M tubes, Figs. 8 and 9, the beam F is displaced between two parallel electrodes, L and P, which are at a distance d and between which a voltage V is applied; the beam is, therefore, submitted to an electric field which draws it toward the positive electrode formed by the delay line L. A magnetic field B is applied perpendicularly to the electric field and transversely to the direction of beam travel. Under these conditions, the electron flow is displaced perpendicularly to the electric and magnetic field and the average electron velocity is In the M tubes, two components of the R.F. field come into play in the interaction mechanism. Since the average electron velocity is perpendicular to the electric field, the longitudinal component of the RF. field causes displacement of the electron toward the delay line, when it is in a direction such that it decelerates the electron. The transverse component of the RF. field accelerates or decelerates, according to its direction, the electron velocity parallel to the line, and brings the electrons into a posi tion such that they are slowed down by the longitudinal component.

The 0" tube of Fig. 6 and the M tube of Fig. 8 are forward wave amplifiers. It will be noted that each of these tubes has an attentuation A, which may be either localized or distributed, located intermediately of the delay line L. The role of this attenuation is to avoid the self oscillations caused by successive reflections at the output S and the input N due to an imperfect match between the delay line and the input and output lines. These self oscillations can appear over the entire frequency band where the tube has a substantial gain. This can more easily be understood by considering that an input signal when impressed through N upon the delay line is amplified by the prolonged interaction of the beam F and the electromagnetic wave energy as it travels toward the output end of the tube. A portion of the amplified energy arriving at the output end, due to an imperfect match, is reflected toward the input end where it is again reflected and is then moving in the proper direction for amplification. By this process, the reflected energy is successively amplified and the tube consequently breaks into oscillation. In order to prevent the energy reflected at the output end from initiating oscillations, attenuation A is introduced intermediately of the delay line to absorb the reflected energy. The attenuation also acts upon the energy traveling in the forward direction and, hence, reduces the gain of the amplifier to a considerable extent. For example, if it is desired to construct an amplifier having a gain of at least 20 db for operation under conditions where the load may introduce a mismatch, so that 10 db of attenuation must be utilized to prevent oscillation of the amplifier, then a tube having a total gain of 30 db must be built. The tube of Fig. 7 and the M tube of Fig. 9 are backward wave oscillators and are characterized by the fact that energy is extracted at the end of the delay line L near the gun G, and that the opposite end of the line is provided with an attenuation A which terminates the line in its characteristic impedance. Whatever the load, this absorber prevents the circuit from presenting resonance frequencies. Since the energy reflected by the load does not satisfy the conditions for interaction, in eifect, it does not act on the beam, and it is completely absorbed by the attenuation A so that the internal operation of the tube is not affected. Hence, the oscillation frequency and the power yielded by the beam are independent of load. The insensitivity of the oscillator frequency to variations in the load constitutes an important advantage which is utilized in the present invention.

Fig. 1 schematically represents the invention applied to an M type tube. The tube may be considered as constituted by two sections separated by a transition region, one of the sections being designated a backward wave oscillator section, and the other being denoted a forward wave amplifer section. A delay line 10 having an interdigital periodic structure is shown extending substantially throughout the entire length of the tube envelope. The tube envelope 24 is preferably fabricated of a non-magnetic metallic material and is provided with insulative seals at various convenient locations through which electrical connections are made to the elements housed in the envelope. A planar electrode 11, known as a sole, is positioned in the oscillator section paralell to the delay line structure and spaced therefrom by a distance d A variable voltage source, here indicated by battery 12, establishes an electric field in the interaction space bounded by the delay line and the sole, the latter being biased to a negative potential with respect to the delay line or anode 10. In the amplifier section a separate sole 13 is positioned at a distance d parallel to the delay line 10. A second electric field is established in the interaction space bound by line 10 and sole 13 by a voltage source 14 which biases the sole 13 negative with respect to the delay line. An electron gun, symbolically indicated by an electron-emitting cathode and an accelerating electrode, is provided in each section of the tube, the cathode 15, and positively biased accelerating electrode 16, providing a beam of electrons 17 for the oscillator section, the cathode 18, and biased accelerating electrode 19, providing a beam of electrons 20 for the amplifier section. The electron beam 17 is injected into the oscillator interaction space along an equipotential surface V defined by at a velocity v which is substantially equal to the phase velocity v of a reverse wave, that is, a wave with energy velocity v directed toward the right of the figure and phase velocity v directed toward the left. A magnetic field B directed out of the plane of the drawing and uniform throughout the interaction space of the oscillator section is established by any convenient means (not shown), e.g., an electromagnet. When the current of beam 17 is increased above a critical value, oscillations commence at a wavelength determined by the point on the dispersion curve where V =V for a reverse wave. V is the average translational electron velocity and is equal to E /B in the oscillator section. The continuous field E between anode 10 and sole 11 is equal to where V is the voltage provided by source 12. It follows, therefore, as V is varied, the electron velocity varies, and the wavelength of oscillation in the oscillator section varies accordingly. In the oscillator section the delay line 10 is terminated in its characteristic impedance by an attenuation 21. This attenuation may take the form of an iron coating adhering to the terminal digits of the line or a lossy material inserted in the spaces between the terminal digits. This attenuation prevents the delay line from presenting resonance frequencies by absorbing energy reflected toward the attenuation end of the line. As previously noted in connection with backward wave oscillators, the reflected energy is completely absorbed and the internal operation of the oscillator section is not affected. Now the microwave energy generated in the oscillator section and traveling with a velocity V toward the right traverses the transition region and enters the amplifier section. The velocity of beam 20 in the amplifier section is determined (1) by a magnetic field B directed into the plane of the drawing and uniform throughout the amplifier section interaction space, and (2) by the continuous field E between anode 10 and sole 13, where E; is equal to V being the voltage of source 14.

The velocity of beam 20 is established at a value that causes the beam to interact with a forward wave space component of the wave energy entering that section from the transition region. The amplifier section, operating in the normal manner of a forward wave amplifier, hence amplifies the oscillatory energy generated in the oscillator section, and power is extracted from the tube by means of an output coupling 26 located adjacent the terminus of the delay 10 in the amplifier section. It will be noted that each end of the delay line carries a protuberance 22, 23 which functions as a collector for the respective beams.

It is important to observe that the amplifier section does not have any attenuation inserted in its portion of delay line 10 for the excellent reason that it is unnecessary. Any energy reflected by the output coupling and by a variable load simply travels along the delay line to the attenuation and is there absorbed. This action does not affect the frequency of the oscillator. In addition, since the input to the amplifier section is through the delay line, a perfect coupling match is assured.

The backward wave oscillator section may be tuned through a wide frequency range by varying the voltage V This can be better understood by reference to the dispersion curve of Fig. 4. It will be assumed that the backward wave oscillator section utilizes the fundamental wave k=0, and that the amplifier section utilizes the harmonic k=+1. Since any tangent to the curve k=0 cuts the ordinate in the region, the fundamental wave is accordingly an inverse wave. By similar reasoning, harmonic k=+1 is a forward wave. When the voltage V is set at a value such that the frequency of the oscillation has a wavelength A, corresponding to point a on curve k=0, it is desirable for optimum operation to set voltage V of source 14, so that the amplifier operates in the region of point 0 on curve k=+1. If the frequency of the oscillator is changed, so that the wavelength becomes A corresponding to point d on curve k=0, then the voltage V of the amplifier should be set so that operation is in the vicinity of point f of curve k=+1. In order to automatically adjust voltage V to a proper value when the frequency of the oscillator section is changed, the variable voltage sources 12 and 14 are ganged by means of a connection,

which may be mechanical or electrical, indicated by link 25. It should be noted that V need not, and usually will not, vary as a linear function of V so that a nonlinear linkage or electrical function generator may be required.

In the schematic drawing of Fig. 1, the distances d and d; are indicated to be equal, but this is for convenience of symmetry only, and those distances may very well be made unequal without any adverse operational efiects. As a matter of interest, since the electric field in the oscillator section is a function of voltage V and distance d and the electric field in the amplifier section is a function of voltage V and distance d the distances al and d may be selected to cause V and V to be equal, so that only one power supply is necessary to establish both electric fields.

With regard to the transition region of the tube shown in Fig. 1, the extent of that region will be governed by the strength and fringing effects of the magnetic fields B and B The field B is directed counter to the direction of field B and hence in the transition region the fields are in opposition. The transition region should be of sufficient extent to insure that neither magnetic field has any appreciable etfect upon the other within its own domain.

The rectilinear form of the invention shown in Fig. 1 may be modified by employing a circular delay line and sole electrodes in the form of circular sectors. Fig. 2 diagrammatically depicts the modified embodiment. In that figure a housing 30 is shown enclosing a circular delay line 31 of the interdigital type. In the backward wave oscillator section, the line is terminated in its characteristic impedance by an attenuation 32, which may take the form of an iron coating cohering to several of the digits of the line. Sole 33 of the backward wave oscillator is a sector of a circle concentric with the delay line. Sole 34 of the forward wave amplifier section is also a sector of a concentric circle. Cathode 35 and accelerating electrode 36 provide an electron beam 37 which traverses the interaction space of the oscillator section and is absorbed by a collector electrode 39. The collector electrode 39 is a thick metallic block capable of quickly dissipating heat. The collector may be simply an extension of delay line 31 or it may be an independent structure maintained at an electrical potential close to that of the delay line. The interaction space of the forward wave amplifier is transited by an electron beam 40 furnished by cathode 41 and accelerating electrode 42. The output from the tube is obtained adjacent the terminal end of the delay line in the amplifier section through an output coupling 43. While voltage sources are not indicated in Fig. 2, it is to be understood that potentials are established upon the various electrodes and guns in the manner indicated in Fig. 1. A magnetic field B directed out of the plane of the drawing and uniform throughout the interaction space of the oscillator section, is established by any convenient means, such as an electromagnet. An oppositely directed magnetic field B is established uniformly throughout the amplifier section interaction space. The transition region should be of sufficient extent to insure that neither magnetic field has any appreciable effect upon the other within its own domain.

Fig. 3 illustrates the invention embodied as an type tube. The tube, again, may be viewed as constituted by a backward wave oscillator section, a forward wave amplifier section, and a transition region separating the two sections. A delay line 50 having an interdigital periodic structure is shown extending substantially throughout the entire length of the tube envelope 51. The envelope, in this instance, is indicated as made of an insulative material, such as glass, although a metallic envelope could have been employed without material modification of the internal elements. The delay line at one end is terminated by an attenuation 52 which pre-.

vents microwave energy from being reflected from that end of the line. At its opposite end, the delay line is provided with an output coupling 53 for extracting wave energy from the tube. It is understood that the attenuation end is located in the backward wave oscillator section of the tube. Two electron guns are provided in the tube envelope. The guns are well shielded and separated by a transition region. Gun 54 projects an electron beam 55 toward the collector 56 situated at the end of the oscillator section. A variable voltage source 57 connected between gun 54 and delay line 50 impresses a positive potential on the delay line. The collector 56 is similarly maintained at an appropriate positive potential by source 57. By adjustment of variable source 57, electron beam 55 is caused to interact with a backward wave space component, whereby oscillations are maintained in this section of the tube. The frequency of oscillation may be changed by altering the velocity of beam 55. The energy of the backward wave originating in the oscillator section propagates in the delay line through the transition region and enters the amplifier section.

In the amplifier section an electron gun 58 projects a beam 59 toward collector 60. The velocity of beam 59 is established, by means of variable voltage source 61, at a value such that the beam interacts with a forward wave space component of the wave energy entering from the transition region. The oscillatory energy generated in the oscillator section is thus coupled by the delay line into the amplifier section where that energy is enhanced and extracted from the tube at the output coupling 53.

Variable voltage sources 57 and 61 are indicated to be ganged by a connecting link 62. For reasons previously elucidated, it is desirable to have voltage source 61 vary in a definite relationship with the variation of voltage source 57. The link may be an electrical or mechanical mechanism designed to provide the required function.

An electromagnetic coil 63 closely surrounds the tube envelope and establishes a longitudinal magnetic field therethrough. That magnetic field assures focusing of beams 55 and 59 during transit through their respective interaction regions. In lieu of coil 63 any other appropriate means may be employed to focus the electron beams during travel in the interaction regions.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is, accordingly, desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. An electronic tube comprising an elongated delay line having a periodic structure, means for projecting a first electron beam into a first region adjacent said delay line, means for causing said first electron beam to travel toward one end of said delay line in proximity to said periodic structure and at a velocity related to the negative phase velocity of a space component of a wave propagating along said delay line whereby energy is transferred from said beam to said wave, means terminating said delay line in its characteristic impedance at that end of said delay line toward which said first electron beam is directed, means for projecting a second electron beam into a second region adjacent said delay line, means for causing said second electron beam to travel toward the other end of said delay line in proximity to said periodic structure and at a velocity related to the positive phase velocity of a space component of said wave propagating along said delay line whereby the energy in said wave is amplified, and output means for extracting said amplified energy.

2. A traveling wave tube comprising an elongated delay line having a periodic structure, attenuation means terminating one end of said delay line for completely absorbing micrgwave energy to prevent reflections therefrom, output means coupled to the opposite end of said delay line for extracting wave energy from said tube, an electron gun for projecting a first electron beam into a first interaction region adjacent said delay 'line, means for causing said first electron beam to travel toward said one end of the delay line in proximity to said periodic structure at a velocity substantially equal to the negative phase velocity of a space component of a wave propagatmg along said delay line whereby energy is transferred from said beam to said wave, means for projecting a second electron beam into a second interaction region adjacent said delay line, said first and second interaction regions being separated by a transistion region containing a segment of said delay line whereby wave energy may propagate from one interaction region into the other, and means for causing said second electron beam to travel toward the other end of said delay line in proximity to said periodic structure at a velocity substantially equal to the positive phase velocity of a space component of a wave propagating from said first region into said second region along said delay line.

3. An electronic tube comprising an elongated delay line having a periodic structure, a first elongated electrode spaced from said delay line and defining with said delay line a first interaction space, means for projecting a first beam of electrons into said first interaction space, means for causing said first electron beam to travel in said first interaction space at a velocity related to the negative phase velocity of a space component of a wave propagating along said delay line, means for terminating said delay line in its characteristic impedance at that end of the delay line toward which said first electron beam is directed, a second elongated electrode spaced from said delay line and defining therewith a second interaction space, means for projecting a second electron beam into said second interaction space in a direction opposite from that of the travel of said first beam, means for causing said second beam to travel in said second interaction space at a velocity related to the positive phase velocity of a space component of a said wave propagating along said delay line whereby the energy in said wave is amplified, and output means for extracting said amplified energy.

4. A traveling wave tube comprising an elongated delay line having a periodic structure, a first elongated sole spaced from said delay line and defining therewith a first interaction space, an electron; gun for projecting a first electron beam into said first interaction space, means establishing crossed electric and magnetic fields in said first interaction space for causing said first electron beam to travel in said interaction space at a velocity substantially equal to the negative phase velocity of a space component of an induced wave propagating along said delay line, attenuation meansterminating said delay line in its characteristic impedance at that end of the delay line toward which said first electron beam is directed, a second elongated sole spaced from said delay line and defining therewith a second interaction space, an electron gun for projecting a second electron beam into said second interaction space in a direction opposite that of the travel of said first beam, means establishing crossed electric and magnetic fields in said second interaction space for causing said second beamito travel at a velocity substantially equal to the positive phase velocity of a space component of said wave propagating along said delay line, said first and second interaction spaces being separated by a transition region, and output means for extracting wave energy from said tube.

5. A traveling wave tube comprising an evacuated envelope housing an elongated delay line having a periodic structure, attenuation means terminating one end of said delay line preventing microwave energy from being reflected therefrom, output means coupled to the opposite end of said delay line for extracting wave energy from said tube, a first elongated sole spaced from said delay line and bounding therewith a first interaction space, an

electron gun for projecting a first electron beam into said first interaction space, means establishing crossed electric and magnetic fields in said first interaction space for causing said first electron beam to travel in said first interaction space toward said one end of the delay line at a velocity substantially equal to the negative phase velocity of a space component of a wave propagating along said delay line, a second elongated sole spaced from said delay and bounding therewith a second interaction space, said first and second interaction spaces being separated by a transition region containing a segment of the said delay line whereby wave energy may propagate from one interaction space into the other, an electron gun for projecting a second electron beam into said second interaction space, and means establishing crossed electric and magnetic fields in said second interaction space for causing said second beam to travel toward said other end of the delay line at a velocity substantially equal to the positive phase velocity of a space component of said wave propagating along said delay line.

6. In a traveling wave tube having a single slow wave structure, a backward wave oscillator section including a first portion of said slow wave structure and first beamforming and directing means for projecting a first beam of electrons along said first portion of said srtucture in one direction, and a forward wave amplifier section including a second portion of said slow wave structure and second beam forming and directing means for projecting a second beam of electrons along said second portion of said structure in a direction opposite to said one direction.

7. In a traveling wave tube having a single slow wave structure, a backward wave oscillator section including a first portion of said slow wave structure and first beamforming and directing means for projecting a first beam of electrons along said first portion of said structure in one direction, and a forward wave amplifier section in cluding a second portion of said slow wave structure and second beam forming and directing means for projecting a second beam of electrons along said second portion of said structure in a direction opposite to said one direction, said slow wave structure having attenuation introduced only at the end thereof toward which said first electron beam is projected.

8. In a traveling wave tube oscillator having a single slow wave structure, a backward wave oscillator section including a first portion of said slow wave structure and first beam-forming and directing means for projecting a first beam of electrons along said first portion of said structure in one direction, and a forward wave amplifier section including a second portion of said slow wave structure and second beam forming and directing means for projecting a second beam of electrons along said second portion of said structure in a direction opposite to said one direction, said backward wave oscillator section including attenuating means disposed adjacent the end of said first portion of said structure toward which said first electron beam is projected, said forward wave amplifier section having output coupling means disposed adjacent the end of said second portion of said structure toward which said second electron beam is projected.

References Cited in the file of this patent UNITED STATES PATENTS 2,680,825 Warnecke et al June 8, 1954 2,687,777 Warnecke et al Aug. 31, 1954 2,694,783 Charles Nov. 16, 1954 2,702,370 Lerbs Feb. 15, 1955 2,730,647 Pierce Jan. 10, 1956 2,814,756 Kenmoku Nov. 26, 1957 2,824,256 Pierce et a1. Feb. 18, 1958 FOREIGN PATENTS 699,893 Great Britain Nov. 18, 1953 1,080,027 France May 26, 1954 

